1. Field of the Invention
The present invention relates to a method of automatically determining the posture of an imaged human body in a medical imaging system such as a radiation imaging system.
2. Description of the Prior Art
A certain phosphor, when exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays, or ultraviolet rays, stores a part of the energy of the radiation. When the phosphor exposed to the radiation is exposed to stimulating rays such as visible light, the phosphor emits light (stimulated emission) in proportion to the stored energy of the radiation. Such a phosphor is called a stimulable phosphor.
There has been proposed a radiation image recording and reproducing system employing such a stimulable phosphor. In the proposed radiation image recording and reproducing system, the radiation image information of an object such as a human body is recorded on a sheet having a layer of stimulable phosphor, and then the stimulable phosphor sheet is scanned with stimulating rays such as a laser beam to cause the stimulable phosphor sheet to emit light representative of the radiation image. The emitted light is then photoelectrically detected to produce an image signal that will be recorded as a visible image of the object on a recording medium such as a photographic material or displayed as a visible image on a CRT or the like (see Japanese Laid-Open Patent Publications Nos. 55-12429 and 56-11395, for example).
The radiation image recording and reproducing system is highly advantageous over conventional radiographic systems employing silver-salt photographs in that images can be recorded in a very wide range of radiation exposure. More specifically, it is known that the amount of light emitted from stimulable phosphor upon exposure to stimulating rays is proportional to the amount of radiation to which the stimulable phosphor has been exposed, in a highly wide range. Even if the amount of radiation exposure varies greatly under various imaging conditions, a read-out gain is set to a suitable level, and the amount of light emitted from a stimulable phosphor sheet is read and coverted into an electric signal by a photoelectric transducer. The electric signal is processed to produce a visible radiation image which is recorded on a recording medium such as photographic material or displayed as a visible image on a CRT or the like. By selecting a suitable read-out gain setting, the radiation image can be obtained which is not affected by variations in the amount of radiation exposure.
In the radiation image recording and reproducing system, in order to eliminate influences due to varying imaging conditions or obtain a radiation image which can well be observed, "recording information" indicating either a recording condition in which the radiation image information is recorded on a stimulable phosphor sheet or a recording pattern determined by the area of an object to be imaged such as a chest or a stomach, and an imaging process such as a simple imaging process or a contrast radiographic process, is reviewed prior to the output of a visible image to be observed. Then, the read-out gain is adjusted to a suitable level based on the recording information, and a recording scale factor is determined in order to optimize the resolution according to the contrast of the recording pattern. Where the image signal which is read out is processed such as for gradation processing, image processing conditions should be optimized.
One known process of reviewing recording information of a radiation image prior to the output of a visible image is disclosed in Japanese Laid-Open Patent Publication No. 58-67240. According to this known process, stimulating light having a level lower than the level of stimulating light to be applied in a "main reading mode" for obtaining a visible image to be observed is used to read, in a "preliminary reading mode", the recording information of a radiation image stored on a stimulable phosphor sheet prior to the main reading mode. In the preliminary reading mode, the condition in which the radiation image is recorded can roughly be understood. For effecting the main reading mode, the read-out gain is suitably adjusted, and the recording scale factor is determined, or image processing conditions are selected, on the basis of the information obtained in the preliminary reading mode.
According to the above conventional method, the recording condition and recording pattern of the radiation image information recorded on the stimulable phosphor sheet can be known prior to the main reading mode. Therefore, even if a reading system having a very wide dynamic range is not relied upon, a radiation image that can well be observed can be produced by adjusting the read-out gain to a suitable level and determining the recording scale factor based on the recording information, and processing an electric signal generated in the main reading mode according to the recording pattern.
Once the reading conditions and/or the image processing conditions of the radiation image information are thus determined, the densities of areas of interest in reproduced images of an object may vary from each other when the object is imaged at different postures. More specifically, in order to diagnose the thoracic vertebra of a patient, the chest is imaged from its front side as shown in FIG. 2A of the accompanying drawings and from a lateral side thereof as shown in FIG. 2B. When the chest is imaged from its front side as shown in FIG. 2A, the area of interest or the thoracic vertebra K overlaps the mediastinum which is less permeable to radiation. Therefore, the amount of radiation stored in the area of the stimulable phosphor sheet corresponding to the thoracic vertebra is low, and the amount of light which will later be emitted from this area is also low. When the chest is laterally imaged as shown in FIG. 2B, the thoracic vertebra K lies over the lung P which is more permeable to radiation. Consequently, the amount of radiation stored in the area of the stimulable phosphor sheet corresponding to the thoracic vertebra is high, and the amount of light which will later be emitted from this area is also high. Since the maximum value Smax and the minimum value Smin of an image signal read from the stimulable phosphor sheet remain substantially unchanged regardless of whether the chest is imaged from its front side or lateral side, the reading conditions and/or the image processing conditions which are determined on the maximum value Smax and the minimum value Smin are substantially the same when the chest is imaged from its front side and lateral side. When a radiation image is reconstructed in the main reading mode under these reading and/or image processing conditions, therefore, the imaged thoracic vertebra is of a relatively low density when it is imaged front its front side, and of a relatively high density when it is imaged from its lateral side.
The same problems may be caused for other reasons than those described above. For example, in order to diagnose a joint J in a foot, the foot is imaged from its front side as shown in FIG. 11A and from a lateral side thereof as shown in FIG. 11B. When the foot is imaged from its front side as shown in FIG. 11A, the heel Y overlaps the long bones from the instep to the toes which are less permeable to radiation. Therefore, the amount of radiation stored in the area of the stimulable phosphor sheet corresponding to the heel is low. The amount of light which will later be emitted from this area is lower than the amount of light which will be emitted from the area of interest or the joint J. When the foot is laterally imaged as shown in FIG. 11B, the heel Y does not lie over the instep and the toes of the foot. Consequently, the amount of radiation stored in the area of the stimulable phosphor sheet corresponding to the heel as imaged from the side is higher than the amount of radiation stored in the area of the heel as imaged from the front. The amount of light which will later be emitted from the area of the heel as imaged from the side is substantially the same as the amount of light that will be emitted from the area of the joint J as imaged from the side. Since the minimum value Smin of an image signal read from the stimulable phosphor sheet for the front foot image is lower than the minimum value of an image signal read for the side foot image, when images are read and reproduced under the reading conditions and/or the image processing conditions which are determined on the maximum value Smax and the minimum value Smin, the imaged foot joint is of a relatively high density when it is imaged from its front side, and of a relatively low density when it is imaged from its lateral side.
It may be possible to dispense with the preliminary reading mode, and establish image processing conditions appropriately based on an image signal read in the main reading mode. However, the above drawbacks are also experienced with such an alternative.
To solve the aforesaid problems, it has been customary to enter information indicating what posture the object is taking while it is being imaged, into an image reading device or an image processing device, when radiation image information is read from a stimulable phosphor sheet, and to establish reading conditions and/or image processing conditions based on the entered posture information.
However, it has been highly tedious and timeconsuming to enter posture information each time radiation image information is to be read from a stimulable phosphor sheet. In addition, the operator may enter wrong posture information in error.